<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <form id="f1">
        <input type="text" id="username"/>
        <input type="password" id="pwd"/>
        <input type="submit" value="登录" id="sub"/>
    </form>
</body>
<script>
    var f1= document.getElementById("f1");
    var username= document.getElementById("username");
    var pwd= document.getElementById("pwd");
    var sub= document.getElementById("sub");
    username.onblur=function(){
        var val=username.value;
        if(val.length<3){
            alert("用户名过短");
        }

    }
    pwd.onfocus=function(){
        var val=pwd.value;
        if(val.length<3){
            alert("密码过短");
        }
    f1.onsubmit=function(){
        alert("表单提交");
        }

    }

/*
1.function  funName(x,y){……}
funName(x,y,z);funName(x);
var result=funName(1,2); return;
2.(function(username){
    console.log(userName);
})("zhangsan");
var add=function(x,y){
    return x+y;
}
add(1,2);
dom object.eventName=function(){}
onclick onmouseover onmouseout onblur onfocus onsubmit onselect onmousemove
*/
</script>
</html>